Aluminum pigments are widely used in coatings, inks, plastics, and cosmetics to bring metallic effects. In order to achieve colored metallic effects, generally, aluminum pigments are used in conjunction with color pigments.
Colored metallic effects in coating applications can be achieved via a multilayer approach. In this approach, a layer of paint/ink containing metallic pigment is applied onto a substrate first, followed by a coating(s) of color pigments. While vivid colored metallic effects can be achieved, multilayer approaches are often coupled with high manufacturing costs, as supplemental equipment is necessary to accommodate the additional manufacturing steps. Further, the extra application procedures limit the product turn-over time. Moreover, considerations have to be given to address the interlayer interactions, such as wetting and adhesion during and after manufacturing.
To overcome such manufacturing complexities and high costs associated with the multilayer approach, extensive efforts have been made to achieve colored metallic effects with a one layer system. In this approach, colored pigments are mixed with metallic pigments in a one resin solution system before being applied. However, there are various challenges with the one layer approach and the desired effects using this approach are very difficult to achieve. For example, aluminum pigments have excellent hiding power, and therefore, a high concentration of color pigments has to be used with the aluminum pigments to achieve reasonable color strength. Since the density of the aluminum pigments is much higher than that of typical color pigments, resins, and solvents, the aluminum pigments tend to separate quickly from the rest of the components within the paints/inks. Moreover, while color pigments adopt various shapes and sizes ranging from a few nanometer to hundreds of nanometers, aluminum pigments mostly are platelet like and have sizes ranging from a few micrometers to hundreds of micrometers. These differences in physical parameters make colorant and aluminum pigment behave quite differently in a paint/ink system, thereby preventing uniform coating formation during the application processes.
Efforts have been made to develop aluminum pigments that have color pigments attached to them so as to alleviate the separation of color pigment from the aluminum substrates in the one layer system. For example, U.S. Pat. No. 5,037,475 utilizes thermally polymerizable carboxylic acid molecules containing at least two carboxylic acid groups to promote the absorption of organic pigments onto aluminum substrates. Polymer coatings are formed on the colored aluminum pigments upon the polymerization of the unsaturated carboxylic acid and a crosslinker. In U.S. Pat. No. 5,558,705, color pigments are first physically absorbed onto the surface of aluminum substrates. A coupling reagent is then used to modify the surface of the aluminum substrates and provides binding sites for subsequent polymer networks formed via solution polymerization. U.S. Pat. No. 5,912,283 uses a specific surface-treatment agent to coat color pigments and promote the adhesion between color pigments and the aluminum substrates. Polymer encapsulation is then applied to stabilize the attachment of color pigments on aluminum substrates. In these cases, polymerization is initiated in solution with the presence of crosslinkers to form a polymer network to stabilize the color pigments onto their surfaces. However, colored aluminum pigments manufactured using these approaches suffer from either heavy aggregation or heavy separation of color pigments from the aluminum substrates. As a result, despite tremendous market interests, commercial colored aluminum pigments, such as Silberline's Silbertone® and Showa's Friend Color™, have achieved little market significance.